Activating a volume group without a quorum of disks in the volume group being active

ABSTRACT

A system, computer program product and method for activating a volume group without a quorum of disks in the volume group being active. A first node, e.g., main server, may send a meta data identifier to a second node, e.g., backup server, upon completion of updating the meta data associated with a plurality of disks in a particular volume group shared by both first and second nodes. If the first node becomes inoperative, e.g., crashes, after sending the meta data identifier to the second node, then the second node may take over the functions of the first node by activating one or more volume groups shared by both the main and backup nodes. The second node may activate a particular volume group by identifying a single disk that is active in that particular volume group based on the meta data identifier associated with that particular volume group.

TECHNICAL FIELD

[0001] The present invention relates to the field of logical volumemanagement of data storage resources in servers, and more particularlyto activating a volume group without a quorum of disks in the volumegroup being active, i.e., possessing valid meta data.

BACKGROUND INFORMATION

[0002] A server in a network system may employ a disk storage unit forstoring data employed by the server. The disk storage unit may storevarious types of information such as the operating system under whichthe micro-processor operates, different application programs that arerun by the system and information that is created and manipulated by thevarious application programs.

[0003] The task of allocating disk storage space in the system isgenerally the responsibility of the operating system, e.g., AIX™, UNIX™.The operating system may logically subdivide the disk storage unit intofile systems comprising smaller storage disks. Such systems, in aconventional manner, are employed to store data files, executableprograms, and the like. A characteristic of such systems is that it isoften difficult to increase the size of a file system after the systemis in use and the need for increased size becomes apparent. Nor is iteasy to reduce the size of a file system in order to free up datastorage space.

[0004] These inadequacies may be addressed by the operating system bywhat is commonly referred to as a Logical Volume Manager (LVM). The LVMmay include code incorporated in the operating system kernel that runsabove the traditional physical device drivers. The LVM may divide thedisk storage space into one or more disks that are commonly referred toas physical volumes (PV's). A physical volume group (PVG) is a namedcollection of a plurality of physical volumes (PVs), e.g., 128 physicalvolumes (PVs). The physical volumes (PVs) exclusively owned by arespective physical volume group (PVG) need not be of the same type orsize. Each physical volume (PV) within a physical volume group (PVG) hasa unique identity within the PVG. The physical volume group (PVG) mayprovide for portability of physical storage across systems and containsattributes that are common to all objects within the PVG.

[0005] Within each physical volume group (PVG), all the constituentphysical volumes (PVs) are logically subdivided into physical clusters(PCs) representing a number of equally sized contiguous units of storagespace. A physical cluster (PC) may be the smallest unit of disk spaceallocation, and is a contiguous space on a physical volume (PV).

[0006] A logical volume (LV) may be defined by the LVM to be within aphysical volume group (PVG) that constitutes a named linear addressspace comprising an extensible collection of physical clusters (PCs). Alogical volume (LV) may exist within only a single physical volume group(PVG) but the physical clusters (PCs) assigned to the LV may come fromone or more of the physical volumes (PVs) in the PVG. Hence, theprincipal function of a logical volume (LV) may be to provide theabstraction of an extensible, reliable disk volume that encompasseslogical areas of storage that are larger than individual physicalvolumes (PVs).

[0007] The LVM may further be configured to generate what is commonlyreferred to as “meta data” for each disk in a physical volume group(PVG). Meta data may be system configuration information that may beused to identify the physical volume group (PVG) associated with thedisk as well as the other disks in the physical volume group (PVG) andthe logical volumes (LVs) allocated from physical storage locationswithin the PVG.

[0008] A network system may comprise one or more clients, e.g., user'scomputer, coupled to a host, e.g., server. Clients are the requestingmachines, i.e., send requests to the server, and the server is thesupplying machine, i.e., supply information to clients. If the servercrashes then the clients coupled to the server may no longer have theability to communicate with the server. Subsequently, a server may havewhat is commonly referred to as a backup server sharing the samephysical volumes (PVs) and physical volume groups (PVGs). If the mainserver that is coupled to the clients crashes, then the backup servermay take over the functions of the main server that crashed. Forexample, the backup server may take over the application that wasrunning on the main server. In order for the backup server to take overthe application that was running on the main server, the backup servermust first activate, i.e., start, the shared physical volume groups(PVGs).

[0009] A particular shared physical volume group may be activated by apolicy of ensuring that all of the disks in that PVG comprise valid metadata. The backup server may determine whether or not the meta data isvalid based on an identifier, e.g., time stamp, sent from the mainserver. Once the particular PVG becomes active, the PVG may remainactive as long as all of the disks in that PVG remain active, i.e.,possesses valid meta data. Disks may become inactive, i.e., possessinvalid meta data, for a variety of reasons such as write errors due toa defective disk. When a disk becomes inactive it may consequently causethe PVG to become deactivated. That is, the PVG may not remain activatedsince not all of the disks are active.

[0010] Subsequently, a new policy was developed that required a quorum,i.e., a majority, of the disks in a PVG to be active, i.e., comprisevalid meta data, in order to activate the PVG as well as to maintainactivation of the PVG. However, requiring a quorum of disks to activatea PVG is a waste of resources.

[0011] It would therefore be desirable to activate a volume groupwithout requiring a quorum of disks in the volume group being active,i.e., possessing valid meta data.

SUMMARY

[0012] The problems outlined above may at least in part be solved insome embodiments by a meta data identifier being sent by the main node,e.g., main server, to the backup node, e.g., backup server, uponcompletion of updating the meta data associated with a plurality ofdisks in a physical volume group (PVG) shared by both the main andbackup nodes. If the main node becomes inoperative after sending themeta data identifier to the backup node, then the backup node may takeover the functions of the main node by activating the one or morephysical volume groups (PVG) shared by both the main and backup nodes.The backup node may activate a particular physical volume group (PVG) byidentifying a single disk out of the plurality of disks in theparticular PVG that is active based on the meta data identifierassociated with that particular PVG.

[0013] In one embodiment of the present invention, a method foractivating a volume group without a quorum of disks in the volume groupbeing active may comprise the step of a first node, e.g., main server,sending a first notification to a second node, e.g., backup server,where the first notification indicates that the first node is about toupdate the meta data associated with a plurality of disks in aparticular physical volume group (PVG) shared by both the first andsecond node. The second node may wait to receive a second notificationfrom the first node indicating that the first node has completedupdating the meta data associated with a plurality of disks in aparticular PVG. If the first node updates the meta data associated withthe plurality of disks in the particular PVG then the first node sendsthe second notification to the second node. The second notification mayinclude a meta data identifier. If the first node becomes inoperative,e.g., crashes, upon sending the second notification, i.e., uponcompleting updating the meta data, then the second node may take overthe functions of the first node. In order for the second node to takeover the functions of the first node, the second node may activate theone or more shared PVGs shared by both the first and second nodes. Thesecond node may activate a particular PVG by identifying a single diskinstead of a quorum, i.e., a majority, of disks that is active in theparticular PVG based upon the meta data identifier associated with thatparticular PVG.

[0014] In one embodiment of the present invention, the meta dataidentifier may be a time stamp. The second node may identify the singledisk out of the plurality of disks in a particular PVG that is active bymatching the time stamp in the meta data identifier associated with thatparticular PVG with the time stamps in the meta data associated with thedisks in that particular PVG. In another embodiment of the presentinvention, the meta data identifier may include a list of one or moreactive disks out of the plurality of disks in a particular PVG. Thesecond node may then activate the particular PVG based on one of the oneor more disks that were indicated to be active in the meta dataidentifier.

[0015] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A better understanding of the present invention can be obtainedwhen the following detailed description is considered in conjunctionwith the following drawings, in which:

[0017]FIG. 1 illustrates an embodiment of a network system configured inaccordance with the present invention;

[0018]FIG. 2 illustrates an embodiment of a node in the network systemconfigured in accordance with the present invention;

[0019]FIG. 3 is a diagram illustrating the mapping of the physical spacein the disk storage unit to the logical space; and

[0020]FIG. 4 is a flowchart of a method for activating a volume groupwithout a quorum of disks in the volume group being active.

DETAILED DESCRIPTION

[0021]FIG. 1—Network System

[0022]FIG. 1 illustrates one embodiment of the present invention of anetwork system 100. Network system 100 may comprise one or more clients102A-C coupled to a node 101A, e.g., server. Clients 102A-C may beconfigured to send requests to node 101A and node 101A may be configuredto supply information to the one or more clients 102A-C coupled to node101A. If node 101A becomes inoperative, e.g., crashes, then a backupnode, e.g., node 101B, coupled to node 101A, may take over the functionsof node 101A. Clients 102A-C may collectively or individually bereferred to as clients 102 or client 102, respectively. Nodes 101A-B maycollectively or individually be referred to as nodes 101 or node 101,respectively. It is noted that network system 100 may comprise anynumber of clients 102 as well as any number of nodes 101, e.g., servers,and that FIG. 1 is illustrative. It is further noted that node 101A maybe coupled to more than one backup node 101, e.g., node 101B. It isfurther noted that the connection between clients 102 and node 101A maybe any medium type, e.g., wireless, wired. It is further noted thatclient 102 may be any type of device, e.g., wireless, Personal DigitalAssistant (PDA), cell phone, personal computer system, workstation,Internet appliance, configured with the capability of connecting to theInternet and consequently communicating with node 101A. It is furthernoted that network system 100 may be any type of system that has atleast one node, e.g., server, at least one backup node, e.g., backupserver, and at least one client and that FIG. 1 is not to be limited inscope to any one particular embodiment.

[0023]FIG. 2—Node

[0024]FIG. 2 illustrates an embodiment of the present invention of node101. Referring to FIG. 2, node 101 may comprise a central processingunit (CPU) 210 coupled to various other components by system bus 212. Anoperating system 240, e.g., AIX, UNIX, runs on CPU 210 and providescontrol and coordinates the function of the various components of FIG.2. Application 250, e.g., program for activating a volume group withouta quorum of disks in the volume group being active as described in FIG.4, runs in conjunction with operating system 240 which implements thevarious functions to be performed by application 250. Read only memory(ROM) 216 is coupled to system bus 212 and includes a basic input/outputsystem (“BIOS”) that controls certain basic functions of server 101.Random access memory (RAM) 214, disk adapter 218 and communicationsadapter 234 are also coupled to system bus 212. It should be noted thatsoftware components including operating system 240 and application 250are loaded into RAM 214 which is the computer system's main memory. Diskadapter 218 may be a small computer system interface (“SCSI”) adapterthat communicates with disk storage unit 220. Disk storage unit 220 maybe configured to comprise a plurality of disks configured to store data.It is noted that the program of the present invention that activates avolume group without a quorum of disks in the volume group being activeas described in FIG. 4, may reside in disk storage unit 220 or inapplication 250.

[0025] Implementations of the invention include implementations as acomputer system programmed to execute the method or methods describedherein, and as a computer program product. According to the computersystem implementations, sets of instructions for executing the method ormethods are resident in the random access memory 214 of one or morecomputer systems configured generally as described above. Until requiredby node 101, the set of instructions may be stored as a computer programproduct in another computer memory, for example, in disk storage unit220. Furthermore, the computer program product can also be stored atanother computer and transmitted when desired to the user's workstationby a network or by an external network such as the Internet. One skilledin the art would appreciate that the physical storage of the sets ofinstructions physically changes the medium upon which it is stored sothat the medium carries computer readable information. The change may beelectrical, magnetic, chemical or some other physical change.

[0026] As stated above, operating system 240 runs on CPU 210 andprovides control and coordinates the function of the various componentsof FIG. 2. A function of operating system 240 may be to logicallysubdivide the disk storage unit 220 into a plurality of file systemscomprising smaller storage disks by a Logical Volume Manager (LVM). TheLVM may include code incorporated in the operating system kernel thatruns above the traditional physical device drivers. The LVM may dividethe disk storage space into logical space as illustrated in FIG. 3. FIG.3 is a diagram illustrating the mapping of the physical space of thedisk storage unit 220 to the logical space. The LVM may divide the diskstorage space into one or more disks 301A-N that are commonly referredto as physical volumes (PV's) as illustrated in FIG. 3. Physical volumes301 A-N may collectively or individually be referred to as physicalvolumes 301 or physical volume 301, respectively. It is noted that LVMmay divide the disk storage space into any number of physical volumes301. A physical volume group (PVG) is a named collection of a pluralityof physical volumes (PVs) 301, e.g., physical volumes (PVs) 301A-N. Thephysical volumes (PVs) 301 exclusively owned by a respective physicalvolume group (PVG) need not be of the same type or size. Each physicalvolume (PV) 301 within a physical volume group (PVG) has a uniqueidentity within the PVG. The physical volume group (PVG) may provide forportability of physical storage across systems and contains attributesthat are common to all objects within the PVG.

[0027] Within each physical volume group (PVG), all the constituentphysical volumes (PVs) 301 are logically subdivided into physicalclusters (PCs) 302A-L representing a number of equally sized contiguousunits of storage space. A physical cluster (PC) may be the smallest unitof disk space allocation, and is a contiguous space on a physical volume(PV) 301. Physical clusters 302A-L may collectively or individually bereferred to as physical clusters 302 or physical cluster 302,respectively. It is noted that physical volumes (PVs) 301 may belogically subdivided into any number of physical clusters (PCs) 302. Aphysical cluster (PC) 302 may be “active” meaning that it contains validuser data. A physical cluster (PC) 302 may be “stale” meaning that itcontains outdated user data and needs to be refreshed with updated userdata.

[0028] A logical volume (LV) 303 may be defined by the LVM to be withina physical volume group (PVG) that constitutes a named linear addressspace comprising an extensible collection of physical clusters (PCs)302. A logical volume (LV) may exist within only a single physicalvolume group (PVG) but the physical clusters (PCs) 302 assigned to theLV may come from one or more of the physical volumes (PVs) 301 in thePVG. Hence, the principal function of a logical volume (LV) may be toprovide the abstraction of an extensible, reliable disk volume thatencompasses logical areas of storage that are larger than individualphysical volumes (PVs).

[0029] The LVM may further be configured to generate what is commonlyreferred to as “meta data” for each disk 301 in the physical volumegroup (PVG). Meta data may be system configuration information that maybe used to identify the physical volume group (PVG) associated with thedisk 301 as well as the other disks 301 in the physical volume group(PVG) and the logical volumes (LVs) 303 allocated from physical storagelocations within the PVG. In one embodiment, meta data associated witheach disk 301 may be stored at the beginning of each disk 301 in thephysical volume group (PVG).

[0030] Referring to FIGS. 1-3, backup node 101B may share the same oneor more physical volumes (PVs) 301 and physical volume groups (PVGs) asmain node 101A. As stated above, when main node 101A becomesinoperative, e.g., crashes, backup node 101B may take over the functionsof main node 101A. In order for backup node 101B to take over thefunctions of main node 101A, backup node 101B must activate the sharedone or more physical volume groups (PVGs). As stated in the BackgroundInformation section, a particular physical volume group (PVG) may beactivated by backup node 101B by requiring a quorum, i.e., a majority,of disks 301 in that particular PVG to comprise valid meta data, i.e.,the most recent version of meta data. It would therefore be desirable toactivate a shared physical volume group (PVG) without requiring a quorumof disks in that PVG to be active, i.e., possess valid meta data. Amethod for a physical volume group (PVG) without requiring a quorum ofdisks in that PVG to be active is described below.

[0031]FIG. 4—Method for Activating a Volume Group Without a Quorum ofDisks in the Volume Group Being Active

[0032]FIG. 4 is a flowchart of one embodiment of the present inventionof a method 400 for activating a physical volume group (PVG) without aquorum of disks in that PVG being active.

[0033] In step 401, main node 101A, e.g., server, coupled to one or moreclients 102 may change the allocation of a physical volume group (PVC),e.g., change which disks 301 are part of a physical volume group (PVG),or the physical clusters (PCs) may change state, e.g., active or stale.As stated above, meta data associated with each disk 301 in a particularphysical volume group (PVG) may comprise system configurationinformation that may be used to identify the physical volume group (PVG)associated with the disk 301 as well as the other disks 301 in thephysical volume group (PVG) and the logical volumes (LVs) 303 allocatedfrom physical storage locations within the PVG. Consequently, meta datamay have to be updated by main node 101A when main node 101A, e.g.,server, changes the allocation of a physical volume group (PVG), e.g.,change which disks 301 are part of a physical volume group (PVG), orwhen the physical clusters (PCs) change state, e.g., active or stale.

[0034] In step 402, main node 101A may send a first notification tobackup node 101B and backup node 100B may receive the first notificationwhere the first notification indicates that main node 101A is about toupdate the meta data associated with the plurality of disks 301 in aparticular physical volume group (PVG).

[0035] In step 403, backup node 101B waits to receive a secondnotification from main node 101A indicating that main node 101A hascompleted updating the meta data associated with the plurality of disks301 in a particular physical volume group PVG.

[0036] In step 404, a determination may be made as to whether or notmain node 101A became inoperative, e.g., crashed, prior to updating themeta data associated with the plurality of disks 301 in a particularphysical volume group (PVG). If main node 101A does not becomeinoperative, e.g., crash, prior to updating the meta data associatedwith the plurality of disks 301 in a particular physical volume group(PVG), then main node 101A sends the second notification and backup node101B receives the second notification where the second notificationincludes a meta data identifier, e.g., time stamp, list of one or moredisks 301 associated with the particular PVG that are active, in step405.

[0037] If main node 101A becomes inoperative, i.e., crashes, prior toupdating the meta data associated with disks 301 in a particularphysical volume group (PVG), then backup node 101B takes over thefunctions of main node 101A in step 406. In order to take over thefunctions of main node 101A, backup node 101B must activate, i.e.,start, one or more physical volume groups (PVGs) shared by both mainnode 101A and backup node 101B. It is noted that backup node 101B may benotified that main node 101A became inoperative, e.g., crashed, by anassortment of methods such as an interrupt executed by a higher levelsoftware. It is further noted that other methods of notification wouldbe recognized by an artisan of ordinary skill in the art and that suchembodiments employing such methods would fall within the scope of thepresent invention.

[0038] In step 407, backup node 101B may activate, i.e., start, each ofthe one or more physical volume groups (PVGs) shared by both main node101A and backup node 101B by backup node 101B identifying a quorum,i.e., a majority, of disks 301 in each physical volume group (PVG) thatare active, i.e., possess valid meta data. Backup node 101B may activatea physical volume group (PVGs) shared by both main node 101A and backupnode 101B by identifying a quorum of disks 301 that are valid in thephysical volume group (PVG) since backup node 101B did not receive thesecond notification indicating that main node 101A has completedupdating the meta data associated with each disk 301 in the PVG.

[0039] Referring to step 405, upon receiving the second notificationmain node 101A becomes inoperative, e.g., crashes, in step 408. Uponmain node 101A becoming inoperative, e.g., crashes, backup node 101Btakes over the functions of main node 101A in step 409. In order to takeover the functions of main node 101A, backup node 101B must activate,i.e., start, one or more physical volume groups (PVGs) shared by bothmain node 101A and backup node 101B.

[0040] In step 410, a physical volume group (PVG) shared by both mainnode 101A and backup node 101B may be activated by backup node 101Bbased on backup node 101B identifying a single disk 301 out of theplurality of disks 301 in the physical volume group (PVG) that isactive, i.e., meta data associated with the single disk 301 is valid.The single disk 301 may be identified based on the meta data identifier,e.g., time stamp, list of one or more active disks 301 in the PVG,received in the second notification in step 405. It is noted that themeta data identifier may comprise other forms of information used toidentify active disks and that the above examples are illustrative. Inone embodiment, backup node 101B may identify a single disk 301 out ofthe plurality of disks 301 in a particular physical volume group (PVG)that is active by matching the time stamp in the meta data identifierwith the time stamps in the meta data associated with the plurality ofdisks 301 in the particular PVG. If there is a match, the particulardisk 301 associated with the matched time stamp may be said to beactive, i.e., possesses valid meta data. In another embodiment, backupnode 101B may receive a list of one or more disks 301 out of theplurality of disks 301 in the PVG that are valid, i.e., possess validmeta data, in the meta data identifier. Backup node 101B may thenactivate a physical volume groups (PVG) shared by both main node 101Aand backup node 101B based on one of the one or more disks that areactive. It is noted that each of the one or more PVGs shared by bothmain node 101A and backup node 101B may be activated by identifying asingle disk that is active in each PVG if backup node 101B received thenotification that main node 101A updated the meta data associated withthe plurality of disks 301 in each PVG prior to main node 101A becominginoperative. If backup node 101B did not receive notification that mainnode 101A updated the meta data associated with the plurality of disks301 in any PVG prior to main node 101A becoming inoperative, then backupnode 101B may active those PVGs by identifying a quorum, i.e., amajority, of disks 301 in each of those PVGs that are active as in step407.

[0041] It is noted that method 400 may be executed in a different orderpresented and that the order presented in the discussion of FIG. 4 isillustrative. It is further noted that certain steps may be executedalmost concurrently.

[0042] Although the system, computer program product and method aredescribed in connection with several embodiments, it is not intended tobe limited to the specific forms set forth herein, but on the contrary,it is intended to cover such alternatives, modifications, andequivalents, as can be reasonably included within the spirit and scopeof the invention as defined by the appended claims. It is noted that theheadings are used only for organizational purposes and not meant tolimit the scope of the description or claims.

1. A method for activating a volume group without a quorum of disks insaid volume group being active comprising the steps of: sending a firstnotification of updating data associated with a plurality of disks in afirst volume group shared by a first node and a second node; receiving asecond notification by said second node indicating that said dataassociated with said plurality of disks in said first volume group hasbeen updated, wherein said second notification comprises a dataidentifier; and activating said first volume group by identifying asingle disk with valid data out of said plurality of disks in said firstvolume group based on said data identifier.
 2. The method as recited inclaim 1, wherein said step of activating said first volume group sharedby said first and said second node occurs after said first node becomesinoperative, wherein said first node becomes inoperative after sendingsaid second notification.
 3. The method as recited in claim 1, whereinsaid data is system configuration information.
 4. The method as recitedin claim 1, wherein said data identifier is a time stamp.
 5. The methodas recited in claim 1, wherein said data identifier is an indication ofone or more of said plurality of disks in said first volume group thatcomprise valid data.
 6. The method as recited in claim 1, wherein saiddata associated with said plurality of disks in said first volume groupis updated if the allocation of said first volume group shared by saidfirst and said second node needs to be changed.
 7. A computer programproduct having computer readable memory having computer program logicrecorded thereon for activating a volume group without a quorum of disksin said volume group being active, comprising: programming operable forreceiving a first notification of updating data associated with aplurality of disks in a first volume group shared by a first node and asecond node; programming operable for receiving a second notificationindicating that said data associated with said plurality of disks insaid first volume group has been updated, wherein said secondnotification comprises a data identifier; and programming operable foractivating said first volume group by identifying a single disk withvalid data out of said plurality of disks in said first volume groupbased on said data identifier.
 8. The computer program product asrecited in claim 7, wherein said programming step of activating saidfirst volume group shared by said first and said second node occursafter said first node becomes inoperative, wherein said first nodebecomes inoperative after sending said second notification.
 9. Thecomputer program product as recited in claim 7, wherein said data issystem configuration information.
 10. The computer program product asrecited in claim 7, wherein said data identifier is a time stamp. 11.The computer program product as recited in claim 7, wherein said dataidentifier is an indication of one or more of said plurality of disks insaid first volume group that comprise valid data.
 12. The computerprogram product as recited in claim 7, wherein said data associated withsaid plurality of disks in said first volume group is updated if theallocation of said first volume group shared by said first and saidsecond node needs to be changed.
 13. A system, comprising: a first node;and a second node coupled to said first node, wherein said second nodeis configured to take over the functions of said first node if saidfirst node becomes inoperative, wherein said second node comprises: aprocessor; a memory unit operable for storing a computer programoperable for activating a volume group without a quorum of disks in saidvolume group being active; an input mechanism; an output mechanism; anda bus system coupling the processor to the memory unit, input mechanism,and output mechanism, wherein the computer program is operable forperforming the following programming steps: receiving a firstnotification of updating data associated with a plurality of disks in afirst volume group shared by said first node and said second node;receiving a second notification indicating that said data associatedwith said plurality of disks in said first volume group has beenupdated, wherein said second notification comprises a data identifier;and activating said first volume group by identifying a single disk withvalid data out of said plurality of disks in said first volume groupbased on said data identifier.
 14. The system as recited in claim 13,wherein said programming step of activating said first volume groupshared by said first and said second node occurs after said first nodebecomes inoperative, wherein said first node becomes inoperative aftersending said second notification.
 15. The system as recited in claim 13,wherein said data is system configuration information.
 16. The system asrecited in claim 13, wherein said data identifier is a time stamp. 17.The system as recited in claim 13, wherein said data identifier is anindication of one or more of said plurality of disks in said firstvolume group that comprise valid data.
 18. The system as recited inclaim 13, wherein said data associated with said plurality of disks insaid first volume group is updated if the allocation of said firstvolume group shared by said first and said second node needs to bechanged.